Not in any particular order (apart from the order they come to mind):
Look for ways to rest. Kneebars, fist jams, elbow bars, chin hooks (I've used this once when I couldn't let go with either hand without doing a barndoor). A well known veteran once used a face jam, he shoved his head in a crack and bulged his cheeks as hard as he could, so he could let go with his hands
Try to climb efficiently. Little things like placing your feet accurately, finding less strenuous ways to get up a problem.
Hang with straight arms. Hang on one arm while shaking out the other (apparently lifting it above your head and shaking/waving works - it looks ridiculous though).
Loose weight. I thought it was bollocks, but after gaining a few, I suck even more than I used to.
Your fingers will get stronger over time, don't push them too hard in the beginning. Try to concentrate on fitness and technique, they go well together in any case. When you train, try to climb laps on long routes with lots of jugs, but try to get up it as efficiently as possible. You will later notice that you can hold smaller and less straight forward grips with little effort. You will learn how to not over grip. Basically get as much mileage as you can on as many different styles as you can.
For crack climbing you need to ask someone (preferably as many someones as possible) to show you how to do hand jams, fist jams, finger jams, finger cams, finger stacks, finger locks and what to do with your feet. It's a bit hard to explain without showing.
Watch other people do it and try to figure out what they are doing.
Ask people to give you pointers. Most of the time you won't even have to ask, lots of climbers tend to tell you voluntarily, you just need to listen.
Different angles / rock texture / grip shape and size needs different technique. Try to do them all as much as you can.
Start slowly, especially if you are climbing somewhere / some style you haven't before or not recently and work your way up.
Warm up.
Drink more water.
Relax.